Honestly, this is why people get away with stuff. The longer you wait, the weaker your position feels. It's like when you don't call out a friend for a rude comment right away. Bringing it up months later just seems weird and they act like you're the problem. You did the work, you have every right to get paid. Send a final notice giving them a firm deadline, then be ready to take the next step. Silence is a tactic.